Reciprocating Gas Engines for Biomass Powerplants

Gas engines are a more economical and environmentally friendly choice for low output power, generation using gaseous fuels. This article describes the functioning of gas engines and looks at how they score over gas turbines in the low output range.

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The effort to reduce greenhouse gas emissions and reduce carbon footprint has led to giant strides in the developments in the production and supply of environmentally friendly low cost fuel gases from organic sources. The ideal way to burn these gaseous fuel for power generation is to use a gas turbine. The organic gas production is mostly in the agricultural community and volumes are low calling for a low output power generation usually in kilowatts rather than in megawatts. For low power output applications gas turbine power tends to be very expensive and requires high technology that the agricultural community cannot afford. A simpler, cheaper and greener solution is to use gas engines. This offsets the use of diesel or gasoline engines with their prohibitively high fuel cost and high Carbon and NOx emissions.

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Gas engines are in the same genre as the petrol or gasoline engines. In fact, gas engines were the predecessor of the present day diesel and gasoline engines. Today gas engine generators are available in ranges from 250 KW to 10 MW. Gas engines are available from the simple to the electronically controlled high efficiency engines.

They use natural gas, gas from biomass sources, landfill gases, mine gases and other inorganic gases from various industrial processes. The list is endless.

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How They Work

Gas engines are four stroke engines using the Otto cycle and in principle work exactly like the petrol or gasoline engine. Air and gas mixture ignites at the end of the compression stroke using a spark. Large gas engines use a fuel rich pre-combustion chamber to amplify the sparks energy. Large engines also use turbo-charging to boost power and output.

The difference is in the fuel system. Unlike the carburetion system or fuel injection system in today’s gasoline engines, the gas suction is by venturi effect and mixed in a gas mixer in the suction manifold. Electronic fuel gas valves for individual cylinders give much better load control. The higher capacity engines use electronic controls to control air fuel ratio and load to get the best efficiencies and prevent knocking. Combustion with high excess air or lean burning reduces the gas temperatures resulting in lower NOx emissions. Large engines also use turbo charging to boost outputs and torques.

V shaped cylinder arrangement as in large diesel sets are the normal configuration for large capacity engines. An optimized cooling system for jacket cooling, lube oil, and inlet air increases efficiency.

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The gas engine can be for power generation only or for combined heat and power utilizing the exhaust heat.

Gas turbines can be used in Combined Cycle mode with a higher efficiency.

The high energy density and portability of liquid fuels makes the diesel or gasoline engines more suitable for transport applications.

The gas engine has some specific advantages over gas turbines in the lower power output range.

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10 Advantages of Gas Engines

Fuel Flexibility. The biggest advantage is that gas engines can burn natural gas and a variety of other gases. The limited availability of many of the organic and process gases necessitates the use of natural gas as a standby source. Switching from one source is rather easy in the gas engine. They can burn low calorific value gases, low methane number gases, and gases that have fluctuating fuel composition like landfill gas.

Fuel Pressures. Gas pressure requirement of a gas turbine is high often in the range of 17 to 20 bar. If the supply gas pressure is low, this requires the addition of a gas pump that increases investment and operation costs. Supply of gas from biomass or other organic sources are at low pressures. Gas engines operate with low inlet gas pressures in the range of 200 mbar.

Speed. As an internal combustion engine, gas engines run at much lower speed, 750 or 1500 rpm against the 14000 RPM for smaller gas turbines. There is no need of a gear box. This is much less stress on the rotating elements, combustion path, and bearings, making gas engines cheaper. This also results in a much higher Mean Time Between Outages.

Ambient Effect. Gas turbine output reduces with the increase in ambient temperature. Gas engine output reduces only when the ambient temperature reduces the effectiveness of the cooling system.

Startup. Gas turbines require high power to start-up, to bring the rotor to synchronous speed. This requires a backup power or stand by DG set, or compressed air pressure or gas pressure. Gas engines can crank start from battery power.

Efficiency. In the lower power range, gas turbine efficiencies are in the range of 28 % in the open cycle mode. Present day large capacity gas engines provide greater than 40 % efficiency.

Exhaust Temperature. Gas turbine exhaust temperature is in the order of 450 °C. Unless there are exhaust heat recovery devices, the exhaust stack and ducts design should be for these high temperatures. This will also require a higher stack to avoid heat dissipation problems. Gas engines in the stand-alone mode have much lower gas temperatures in the range of 150 °C.

Emissions. The lower combustion temperature and lean air burning produces less NOx.

Maintenance. Skills required for maintenance follow the well-developed IC engine lines and is thus available in almost all parts of the world. Gas engines are very reliable and run for almost 100 % availability resulting in low costs. Gas turbines require high technology and more time for maintenance.

Cost. All of the above makes gas engine a lot cheaper, with lower capital costs and lower O&M costs.

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For low output power generation with environmentally friendly fuel gases, gas engines are a better option than gas turbines.